The search for and recovery of oil is becoming increasingly difficult as world-wide petroleum reserves decline. In many instances, reserves trapped within certain low permeability formations, such as certain sandstone, carbonate, and/or shale formations, exhibit little or no production, and are thus economically undesirable to develop at current oil and gas prices. Well stimulation is one method that is frequently employed to increase the net permeability of a formation or reservoir, thereby leading to increased production from these wells that have little or no production.
During well stimulation operations, chemicals can be injected into the formation in a process known as well stimulation. Some exemplary known stimulation techniques include: (1) injection of chemicals into the wellbore wherein the chemicals react with and/or dissolve production limiting conditions (e.g., the presence of clays and drilling solids); (2) injection of chemicals through the wellbore and into the formation to react with and/or dissolve portions of the formation, thereby creating alternative flow paths for recoverable hydrocarbons (e.g., acid-fracturing or matrix-fracturing processes); and (3) high pressure injection of water or chemicals through the wellbore and into the formation at pressures that are sufficient to fracture the formation, thereby creating new or additional flow channels through which hydrocarbons can more readily move from the formation into the wellbore.
In certain tight reservoirs, well productivity is typically low, thus making the well non-economical from a standpoint of development. One commonly employed technique for stimulating low productivity wells is hydraulic fracturing, which can involve the injection of fluids, such as high viscosity fluids, into the well at a sufficiently high rate so that enough pressure is built up inside the wellbore to split the formation apart or fractures. The resulting hydraulically induced fracture that is produced as a result of the fracturing can extend from the wellbore deep into the formation.
Sandstone formations can be particularly susceptible to formation damage from formation minerals such as clay and other siliceous deposits. Stimulation methods for these types of formations have typically employed acid or acid-based fluids for the treatment or stimulation due to the ability of the acid or acid-based fluid to readily dissolve both formation minerals and contaminants (e.g., drilling fluids that may be coating the wellbore or that has penetrated into the formation) introduced into the wellbore/formation during drilling or remedial operations.
The removal of formation minerals and other deposits, such as clays, drilling solids, or precipitates, are key concerns in well completion operations. As noted above, the known prior art techniques typically involve acids, often highly concentrated acids such as hydrofluoric acid (HF) and HCl—HF acid mixtures, which are both highly corrosive and can create dangerous operating conditions. In a typical HF based well stimulation, a preflush solution that includes ammonium or potassium chloride is injected into a well, followed by the injection of a well stimulation fluid (either mud based or organic based) that includes HF or an HF/HCl mixture into the formation. After allowing the well stimulation fluid to react with the formation for a predetermined amount of time, an overflush solution that includes HCl and/or brine (which can include ammonium or potassium chloride) is injected into the formation to end the reaction of the stimulation fluid with the formation. In many instances, the reaction of the acid with the formation is rapid, frequently instantaneous, thereby limiting the penetration depth of the acid.
One difficulty that is frequently encountered with traditional sandstone stimulation operations that employ HF as a stimulation fluid is that HF can precipitate into the formation, typically as a fluoride, thereby causing formation damage and limiting the efficiency of the matrix stimulation treatment. The damage caused by fluoride precipitation can be aggravated at the higher temperatures that are typically encountered in the formation. Another difficulty encountered with traditional HF sandstone stimulation is the acid reacts very rapidly with the formation, often instantaneously, thereby limiting the penetration of the acid into the formation. Therefore, it would be advantageous to develop a stimulation fluid and method of using same that reacts at a slower rate with the formation and which does not form precipitants upon reaction with the sandstone formation at high temperatures.